Frost and dew sensor

ABSTRACT

In a frost and dew sensor composed of two thermosensitive resistors, a power source and an arithmetic circuit, frost or dew is detected from the change of a differential temperature between the two thermosensitive resistors as two different currents from the power source are supplied to the respective thermosensitive resistors. In an alternative form, the sensor has a single thermosensitive resistor, which is energized as two different currents are alternately supplied from the power source so that frost or dew is detected by comparison of the time-lags between the two outputs from the thermosensitive resistor.

This is a division of application Ser. No. 07/420,600 filed Oct. 12,1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a frost and dew sensor for use in a defrosterof a refrigerator, air-conditioner and various other industrialappliances.

2. Description of the Related Art

It has been a common knowledge that under certain conditions, thesurface of a heat-exchanger, incorporated in a refrigerator, anair-conditioner or a similar apparatus, is covered with frost and ice.Continued operation of the heat-exchanger covered with frost and icewould remarkably reduce the energy efficiency, which is uneconomical andoccasionally causes a failure or fault.

Heretofore, attempts were made to detect the building-up of frost anddew; some of the proposed detecting means used a resonator, someutilized the change of dielectric constant of an element due to thedeveloping of frost or dew, and others were optical type.

FIGS. 13 through 16 of the accompanying drawings illustrate two priorart sensors each using a resonator; one sensor detects the change ofresonance frequency of a resonator, and the other detects the change ofamplitude of a resonator.

In FIG. 13, a piezo-electric resonator 14 is supported on the uppersurface of a tubular housing 10 via a resilient support 12 and bears apair of electrodes 16a, 16b attached one on each side of the resonator14, and a pair of output terminals 18a, 18b leading from the electrodes16a, 16b, respectively.

FIG. 14 is a circuit diagram of the sensor of FIG. 13, in which theoutput of the resonator 14 is supplied to a resonance-frequencydiscriminator 22 via a resistor R on one side and an amplifier 20 on theother side where the output is amplified as a matter of fact. Then theoutput of the discriminator 22 is taken out to the exterior. Inoperation, as frost or dew develops over the surface of the resonator14, the resonance frequency derived from the resonator 14 variesdepending on the amount of frost or dew built up. When the extent ofchange in resonance frequency climbs over a predetermined value, thissensor discriminates or judges that the resonator 14 has been coveredwith frost or dew.

The sensor of FIG. 15 is of the type in which the developing of frost ordew is detected based on the change of amplitude of a resonator 114.This sensor is identical in basic construction with that of FIG. 13;but, the output of the resonator 114 is supplied to an oscillationamplitude discriminator 124, as shown in FIG. 16. In this sensor, asfrost or dew develops over the surface of the resonator 114, theoscillation of the resonator 114 is restricted depending on the weightof frost or dew grown up. Thus when the extent of change in amplitudeascends beyond a predetermined value, this sensor presumes that thesurface of the resonator 114 has been covered with frost or dew.

FIGS. 17 to 19 are diagrams showing various wave forms of theoscillation outputs of the piezo-electric resonator 114 and of thedetected outputs of the sensors shown in FIGS. 13 through 16.

Specifically, FIG. 17 shows a wave form of the oscillation output of thesensor of FIGS. 13 and 14, indicating that at a time point t₁,concurrently with the developing of frost, the resonance frequencyincreased about two times. FIG. 18 shows another wave form of theoscillation output of the sensor of FIGS. 15 and 16; it can be observedthat concurrently with the developing of frost or dew (time point t₁),the amplitude of the output signal derived from the resonator 114 isreduced.

When any change of the oscillation frequency or amplitude has thus beenfound, the sensor outputs a signal giving a notice that the resonator14, 114 has been covered with frost or dew, in response to whichgenerally a defroster or a dehumidifier is energized.

FIGS. 20 to 22 illustrate another prior art sensor of the type utilizingthe change of dielectric constant to detect frost and dew. FIGS. 20 and21 show the inside structure and outside appearance, respectively, ofthe sensor; a resistor film 230 is coated over the surface of aninsulating substrate 228 on which a pair of comb-shaped electrodes 226,226 is printed. FIG. 22 shows a detector circuit of the sensor; analternating voltage from an alternating signal source 134 is impressedto a detection unit 232 having the construction of FIGS. 20 and 21, andthe output of the detection unit 232 is supplied to an impedancedetector circuit 236, the output terminal of which is connected to anon-illustrated defroster or dehumidifier.

With this prior arrangement, as frost or dew develops over the surfaceof the detection unit 232, the alternating impedance between the twocomb-shaped electrodes 226, 226 varies. When the impedance detectorcircuit 236 detects this change in the impedance, it presumes that thesurface of the detection unit 232 has been covered with frost or dew.

FIGS. 23 and 24 illustrate an optical type of prior art sensor.

Specifically, FIG. 23 shows the principle of operation of the sensorhaving a light-emitting element 338 and a light-receiving element 340;light from the light-emitting element 338 reflects on a reflectionsurface 342 and then strikes on the light-receiving element 340. Asfrost or dew develops over the reflection surface 342, the refractiveindex of the light from the light-emitting element 338 or the angle ofincidence of the light falling on the light-receiving element 340deviates so that the amount of light falling on the light-receivingelement 340 is reduced. When any change of the light amount is thusfound, the sensor makes a judgment that the surface of the reflectionsurface 342 has been covered with frost or dew.

FIG. 24 show the principle of operation of the sensor having an LED(light-emitting diode) 438 and a photodiode 440 receptive of the lightfrom the LED 438. As frost or dew develops on a path of light spanningbetween the LED 438 and the photodiode 440, the amount of light to reachthe photodiode varies. When the extent of change in the light amount iscompared with a reference value in a level discriminator 444 and is thusfound over the reference value, the level discriminator 444 issues anotice that the path of light between LED 438 and the photodiode 440 hasbeen at least partly blocked by frost or dew grown up.

With the foregoing prior arrangement, the following problems areunavoidable so that adequate usefulness cannot be achieved.

Each of the known sensors of FIGS. 13 through 16, in which apiezo-electric resonator is used, tends to operate incorrectly due tothe dust or other foreign matter stuck to the resonator or due tovibrations exerted on the resonator interiorly and exteriorly of thesensor.

In the known sensors of FIGS. 20 through 24, some utilizing the changein dielectric constant and others adopting an optical method, partlysince it is difficult to reduce the detection unit into a compact size,and partly since the circuit structure is too complex, maintenance on aperiodical basis is essential to keep the detection precision at apredetermined level. Accordingly, it is difficult not only to achievereproducibility, but also to reduce the cost of production.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide a frost and dewsensor which is compact in size and hence inexpensive to manufacture andwhich is excellent in both detection precision and reproducibility.

According to a first aspect of this invention, there is provided a frostand dew sensor comprising: a pair of thermosensitive resistors disposedadjacent to each other, each of the thermosensitive resistors beingcapable of generating heat by itself due to a given current suppliedthereto, each thermosensitive resistor having a resistance varyingaccording to the change of temperature of each thermosensitive resistor;a power source for creating a differential temperature between thethermosensitive resistors, the power source including a heat-generatingcurrent source for supplying a heat-generating current to one of thethermosensitive resistors, and a comparative reference current sourcefor supplying a comparative reference current to the otherthermosensitive resistor so as to cause a temperature increase of theother thermosensitive resistor to only a negligible extent with respectto the temperature increase of the one thermosensitive resistor due tothe heat-generating current; and an arithmetic circuit for fetching atemperature at a respective one of the thermosensitive resistors as anoutput voltage corresponding to the resistance of the respectivethermosensitive resistor and for generating a frost-and-dew signalaccording to a difference in output voltage between the pair ofthermosensitive resistors.

In this first arrangement, in air, one thermosensitive resistorreceptive of a current supplied from a heat-generating current source iskept at a temperature higher than the temperature of the otherthermosensitive resistor receptive of a current normally supplied from acomparative reference current source.

When frost or dew develops over these two thermosensitive resistorslocated adjacent to each other, heat radiation, from the onethermosensitive resistor kept at a high temperature due to a relativelylarge current from the heat-generating current source, occurs via thefrost or dew developed over the surface of that one thermosensitiveresistor. This is because either solid (frost) or liquid (dew) has aheat conductivity larger than air, i.e., gas. As a result, thetemperature of the one thermosensitive resistor descends to reduce adifferential temperature with respect to the other thermosensitiveresistor kept at a substantially constant temperature due to a currentsupplied from the comparative reference source.

The arithmetic circuit calculates the differential temperature createdbetween the two thermosensitive resistors; when this differentialtemperature satisfies specified conditions, the arithmetic circuit makesa judgment that the surfaces of the thermosensitive resistors have beencovered with frost or dew, and outputs a frost-and-dew signal.

The arithmetic circuit may include a comparator circuit which iscomposed of two comparators, for example. One of the comparatorscompares the respective output signals of the thermosensitive resistorswith one another, and the other comparator compares the output signal ofthe one comparator with a reference value.

Further, the arithmetic circuit may be equipped with a freezing leveldetector circuit for comparing the output of the other thermosensitiveresistor corresponding to the comparative reference current with apredetermined freezing level reference value to detect only thedeveloping of frost. The arithmetic circuit may also be equipped with adiscriminator for comparing the output of the comparator circuit withthe output of the freezing level detector circuit.

The frost-and-dew signal may be outputted through an output circuitwhere this signal is amplified. The sensor may use a shaft for carryingthereon the individual thermosensitive resistor, and a base supportingthe shaft.

The arithmetic circuit should by no means be limited to the analog type.That is, in an alternative form, the output of the individualthermosensitive resistor may be converted between analog form anddigital form by an A/D converter, then a differential temperature may becalculated by a μ-CPU, and finally a frost-and-dew signal may beoutputted based on the arithmetical result.

The individual thermosensitive resistor may be a thermistor or aresistor made from platinum or nickel.

According to a second aspect of this invention, there is provided afrost and dew sensor comprising: a thermosensitive resistor capable ofgenerating heat by itself due to a given current supplied thereto, saidthermosensitive resistor having a resistance varying according to thechange of temperature of said thermosensitive resistor; a power sourcefor changing the temperature of said thermosensitive resistorperiodically, said power source including a variable constant-currentsource for alternately supplying a heat-generating current and acomparative reference current to said thermosensitive resistor, saidcomparative reference current being such that it causes a temperatureincrease of said thermosensitive resistor to only a negligible extentwith respect to the temperature increase due to the heat-generatingcurrent; and an arithmetic circuit for fetching a temperature of saidthermosensitive resistor as an output voltage according to a resistancecorresponding to the temperature and for generating a frost-and-dewsignal according to a difference between the output voltage during theheat-generating current is being supplied to said thermosensitiveresistor and the output voltage during the comparative reference currentis being supplied to said thermosensitive resistor.

In this second arrangement, the arithmetic circuit may include anamplifier for amplifying the output voltage of said thermosensitiveresistor, during the comparative reference current is being suppliedthereto, a holding circuit for holding the output of said amplifier tooutput the same output with a delay for the duration the comparativereference current is being supplied to said thermosensitive resistor,and a comparator circuit for comparing a differential temperature,between the output of said holding circuit and the output voltage ofsaid thermosensitive resistor during the heat-generating current isbeing supplied, with a predetermined reference value.

Further, the power source may be equipped with a timing circuitincluding an oscillator for producing an alternate timing to alternatethe output of said variable constant-current source between thecomparative reference current and the heat-generating current. Each ofthe amplifier and an output circuit similar to that of the firstarrangement may be equipped with a switch adapted to be energized anddeenergized by the timing circuit.

It is also particularly useful when additional parts similar to those ofthe first arrangement are incorporated in this second arrangement.

To sum up, according to this invention, since the developing of frost ordew is judged in terms of temperature, occurrence of any misdetectiondue to dust or other foreign matter stuck on the surface of a detectionunit can be reduced to a minimum. Further, since at least onethermosensitive resistor, which is small in size and inexpensive, isused, it is possible to achieve a frost and dew sensor which can bemanufactured at a reduced cost and can offer an improved performance.

The above and other objects, features and additional advantages of thisinvention will become manifest to those versed in the art upon makingreference to the detailed description and the accompanying drawings inwhich a variety of preferred structural embodiments incorporating theprinciple of this invention are shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a partial appearance of a frost anddew sensor according to a first embodiment of this invention, in which apair of thermosensitive resistors is located adjacent to each other;

FIG. 2 is a circuit diagram of the sensor of the first embodiment;

FIG. 3 is a characteristic curve graph showing the general relationbetween the temperature and resistance of a thermosensitive resistor;

FIG. 4 is a characteristic curve graph showing the relation between thetemperature or differential temperature and the resistance when no frostor dew exists around the thermosensitive resistors in the firstembodiment;

FIG. 5 is a characteristic curve graph similar to FIG. 4, showing thetypical relation between the temperature or differential temperature andthe resistance when frost or dew has developed over the thermosensitiveresistors;

FIG. 6 is a block diagram of a modified sensor according to a secondembodiment;

FIG. 7 is a block diagram of another modified sensor according to athird embodiment;

FIG. 8 is a perspective view showing a partial appearance of the sensorof the third embodiment;

FIG. 9 is a side elevational view of FIG. 8, showing the sensor with nofrost or dew around;

FIG. 10 is a view similar to FIG. 9, showing the sensor having beencovered with frost or dew;

FIG. 11 is a detail circuit diagram corresponding to FIG. 7;

FIG. 12 is a block diagram showing still another modified sensoraccording to a fourth embodiment;

FIG. 13 is a cross-sectional view of a prior art sensor for detectingfrost or dew by the change in oscillation frequency of an piezo-electricresonator;

FIG. 14 is a circuit diagram of the sensor of FIG. 13;

FIG. 15 is a view similar to FIG. 13, showing another prior art sensorfor detecting frost or dew by the change in oscillation amplitude of apiezoelectric resonator;

FIG. 16 is a circuit diagram of the sensor of FIG. 15;

FIG. 17 is a wave-form graph showing the oscillation output of thesensor of FIG. 13;

FIG. 18 is a wave-form graph similar to FIG. 17, showing the oscillationoutput of the sensor of FIG. 15;

FIG. 19 is a detection output graph corresponding to FIGS. 17 and 18,showing the presence/absence of frost or dew;

FIG. 20 is a plan view of still another prior art sensor in which frostor dew is detected in terms of the change in dielectric factor;

FIG. 21 is a perspective view of the sensor of FIG. 20;

FIG. 22 is a block diagram showing a circuit of the sensor of FIG. 20;and

FIGS. 23 and 24 are schematic diagrams of two other kinds of prior artsensors, showing the structure and the principle of operation of theindividual sensor in which frost or dew is detected in terms of thechange of amount of light reaching a photodiode from an LED.

DETAILED DESCRIPTION

Various preferred embodiments of this invention will now be describedwith reference to the accompanying drawings. A thermistor is used hereas a thermosensitive resistor.

FIG. 1 shows a partial appearance of a frost and dew sensor (hereinafteralso called "sensor") according to a first embodiment of this invention.

In the sensor of the first embodiment, the developing of frost or dewcan be judged or discriminated in terms of a differential temperaturecreated between a pair of thermosensitive resistors. In the illustratedexample, the sensor comprises a pair of thermosensitive resistors 548,550 disposed parallel to each other. These two thermosensitive resistors548, 550 are carried on a pair of shafts 552a and 552b, or 552c and552d, respectively, supported between opposite side walls of a base 546with a constant distance between the shafts. In this embodiment, athermistor having a resistance of 5 kΩ at a B constant of 3350 and 25°C. is used for each of the thermosensitive resistors 548, 550. The heatradiation of the thermistor is 2 mW/ C in air and about 50 mW/°C. inice. Further, the thermistor offers a resistance of 14 kΩ at 0° C.

FIG. 2 shows the entire circuit of the sensor of this invention, inwhich the thermosensitive resistors 548, 550 are grounded at one end,and currents are supplied from a power source Ps to the thermosensitiveresistors 548, 550 via a pair of constant-current variable circuit 554,556, respectively, in which different output current values are preset.

These two constant-current variable circuits 554, 556 jointlyconstitutes a power source 558.

The thermosensitive resistors 548, 550 and the power source 558 areconnected to an arithmetic circuit 560 which outputs a frost-and-dewsignal in a manner described blow.

The respective outputs of the constant-current variable circuits 554,556 are supplied to an operational amplifier 562 where a differentialtemperature between the thermosensitive resistors 548, 550 iscalculated. The calculated differential temperature is supplied to oneinput terminal of a comparator 564 where a judgment is made on whetherthe differential temperature is below a direct-current reference voltageVref₂.

Meanwhile, the comparator 566 monitors a temperature of thethermosensitive resistor 548 and compares the monitored temperature witha direct-current current reference voltage Vref₁ to discriminate whetherthe monitored temperature is below a reference value.

By this comparison, it is possible to detect the developing of frostonly, i.e., apart from the developing of dew, provided that thereference voltage Vref₁ is preset to a value equivalent to that at 0° C.

Each of the comparators 564, 566 issues an output only when either thedifferential temperature value or the monitored temperature value isbelow a reference value. In the illustrated example, this output isnegative logic; a base current flows to a transistor Q₂ to render thetransistor conductive only when these two outputs meet, the transistorQ₂ being included in an output circuit 570.

A transistor Q₁ is deenergized only when the two outputs meet. At thattime a voltage is impressed from a power source Ps₂ to the transistor Q₂via resistances R₇, R₈ to energize the transistor Q₂. As thus energized,the transistor Q₂ outputs an open collector signal giving a notice ofthe developing of frost or dew.

The operation of the sensor according to the first embodiment will nowbe described.

In general, a thermosensitive resistor has a temperature-resistancecharacteristic such as shown in FIG. 3; temperature and resistance arevirtually in reverse proportion to each other.

In FIG. 2, to the one thermosensitive resistor 548, a feeble directcurrent (i.e., comparative reference current) i_(A), such that atemperature increase due to the self-heat-generation is negligible, issupplied from a power source Ps₁ via the constant-current variablecircuit 554. To the other thermosensitive resistor 550, another directcurrent (i.e., heat-generating current) i_(b), such that a temperatureincrease is constant, is supplied from the common power source via theconstant-current variable circuit 556. In this case, assuming that thetemperature increase of the thermosensitive resistor 550 is ΔT, this ΔTis determined from an amount of electric power consumed by thethermosensitive resistor 550 and an amount of heat energy (Qr) radiatedfrom the same thermosensitive resistor.

The relation between an ambient temperature and a thermosensitiveresistor temperature and the relation between an ambient temperature anda differential temperature of the thermosensitive resistors 548, 550,when the thermosensitive resistors 548, 550 are in air, namely, when nofrost or dew exists around the thermosensitive resistors 548, 550, areshown in FIG. 4.

It is a common knowledge that the amount of heat radiation of athermosensitive resistor varies sharply depending on whether thethermosensitive resistor is disposed in gas or it is disposed in solidor liquid. This is because the heat conductivity of solid is larger bytwo figures, compared with that of gas.

Now if the respective temperatures of the two thermosensitive resistors548, 550 are compared to one another in air, the temperature of the onethermosensitive resistor 548 is T₀ +ΔTa, and the temperature of theother thermosensitive resistor 550 is T₀ +ΔTb. Therefore thedifferential temperature is ΔTb-ΔTa as shown in FIG. 4. Here T₀ standsfor air temperature.

On the other hand, if the two thermosensitive resistors 548, 550 arelocated in water or ice (frost), their respective temperatures are T₁+ΔTa' and T₁ '+ΔTb'. Therefore the differential temperature is ΔTb'-ΔTa'as shown in FIG. 5.

However, because the heat conductivity of water or ice is larger thanthat of air, ΔTa'<ΔTa and ΔTb'<ΔTb. As a result, as shown in FIG. 5, thetemperature ΔTb'-ΔTa' is large enough in magnitude, compared to ΔTb-ΔTaas shown in FIG. 4.

When the differential temperature between the two thermosensitiveresistors 548, 550 is reduced to more than a predetermined extent, itcan be regarded as a sign that the surfaces of thermosensitive resistors548, 550 have been covered with frost or dew.

In FIG. 2, the operational amplifier 562 detects the respectivetemperatures of the two thermosensitive resistors 548, 550 to obtain adifference therebetween and then supplies this differential temperaturevalue to one input terminal of the comparator 564.

To the other input terminal of the comparator 564, a reference voltageVref₂, equivalent to the differential temperature between the twothermosensitive resistors 548, 550 when no frost or dew exists aroundthe thermosensitive resistors 548, 550, is impressed. The comparator 564thus makes a discrimination as to whether the output of the operationalamplifier 562 is below the reference voltage value Vref₂.

According to the principles of this invention, yet when the surface of athermosensitive resistor is covered with water (dew) instead of ice(frost), ΔTb-ΔTa is small. Therefore, for detecting frost and dewdistinctly of each other, a comparator 566 may be used to monitor anambient temperature T₀ via the one thermosensitive resistor 548 which isat a temperature very close to the ambient temperature T₀ because theamount of current supplied to the thermosensitive resistor 564 is verysmall.

To the other input terminal of the comparator 566, a reference voltageVref₁ equivalent to the freezing point of water is impressed from thedirect-current source. When this input signal indicative of themonitored temperature from the thermosensitive resistor 548 is smallerthan a reference voltage signal, it can be regarded as a sign that thesurface of the thermosensitive resistor 548 has been covered with frost,not water.

When the detection signal of either comparator 564, 566 is below thereference value, a negative logic output is issued to take a logicaloperation AND by a pair of diodes D₁, D₂ which jointly constitutes adiscriminator 568.

Only when the outputs of the two comparators 564, 566 meet, thetransistor Q₁ is deenergized to stop the current flow from the powersource Ps₂ to the ground.

Consequently a voltage is impressed to the base of the transistor Q₂from the power source Ps₂ via the resistances R₇, R₈, and the transistorQ₂ is thereby energized to issue to a non-illustrated defroster driveunit, for example, an output signal giving a notice of the developing offrost. This output means: "the ambient temperature (i.e., temperature ofthe thermistor 12) is below 0° C., and the surfaces of thethermosensitive resistors are covered with ice (frost)".

The differential temperature ΔTb-ΔTa varies depending on the state ofice covering over the thermosensitive resistors; by selectively varyingthe reference voltage of the comparator 564, it is possible to detectthe developing of ice (frost) with maximum sensitivity.

If it is not necessary to distinguish between frost and dew whendetecting, the comparator 566 and the diodes D₁, D₂ may be omitted.

In the foregoing embodiments, a thermistor is used for eachthermosensitive resistor. However, this invention should by no means belimited to this specific form. The thermosensitive resistor may be aresistor made from platinum or nickel, for example, provided that theresistance-temperature coefficient of that resistor is positive.

Further, a control circuit employing a timer circuit or a microcomputermay be added in the arithmetic circuit.

In the illustrated embodiments, the arithmetic circuit is composed of asingle operational amplifier, a pair of comparators, and transistors.Alternatively, the output voltage of the thermosensitive resistor may beinputted to an A/D converter and then may be processed as digital datawith arithmetic operations by a microprocessor.

FIG. 6 is a block diagram showing a second embodiment having such analternative construction. In this embodiment, a modified arithmeticcircuit 660 is composed of an A/D converter 672 for converting theoutput currents of two thermosensitive resistors 648, 650 from analogform to digital form, a μ-CPU 674 for calculating a differentialtemperature between the thermosensitive resistors 648, 650 from digitaldata outputted from the A/D converter 672, and an output circuit 670 foroutputting a frost-and-dew signal based on the output of the μ-CPU 674.The arrangement of this embodiment can offer the same result as that ofthe first embodiment.

FIG. 7 shows a third embodiment, in which two different-value currents,i.e., a heat-generating current and a comparative reference current froma constant-current variable circuit 776 are alternately supplied to asingle thermosensitive resistor 750 grounded at one end.

In the third embodiment, time-divided different currents i_(A), i_(B)are supplied to the single thermosensitive resistor 750, and thedifferential temperature is detected in terms of the different amountsof heat energy radiated when frost or dew exists around and when nofrost or dew exists around. This detection is achieved by temporarilydelaying one of the two output voltage values of the thermosensitiveresistor 750 to meet with the detection timing signal of the otheroutput voltage value for comparison.

Specifically, one of two alternately selective detection values isdelayed and then supplied to the comparator circuit 762 at the sametiming as the succeeding other detection value.

Here, since the heat-generating current i_(B) is set at a value higherthan the comparative reference current i_(A), a deviation would becreated therebetween during no developing of frost or dew if compared asthey are. To meet the two values with each other, in this embodiment,the comparative reference current i_(A) is amplified by an amplifier778, when detecting the comparative reference current, up to a levelequal to the detection value by the heat-generating current.

As is apparent from FIG. 7, the output of the thermosensitive resistor750 is amplified by the amplifier 778, and then the amplified output isheld for a predetermined period by a holding circuit 780 succeeding tothe amplifier 778.

In this embodiment, this signal is held when the comparative referencecurrent i_(A) is supplied to the thermosensitive resistor 750, and theangle of amplification of the amplifier 778 can be given by the follwingequation:

    α=.sub.B R.sub.0 /i.sub.A R.sub.0 =i.sub.B /i.sub.A

Then the output of the amplifier 778 enters the holding circuit 780.Since two current values are sequentially impressed to this holdingcircuit 780 as described above, the voltages created at thethermosensitive resistor 750 by the two current values cannot becompared. For instance, the voltage created at the amplifier 778, whenthe current value i_(A) is supplied, is temporarily held so that thecomparator circuit 762 can compare this voltage with the voltage createdat the thermosensitive resistor 750 by the supply of the current valuei_(B) after a predetermined period of time.

The comparator circuit 762 compares a differential temperature createdwhen the currents i_(A), i_(B) are supplied to the thermosensitiveresistor 750. On the other hand, to make a judgment on the developing offrost only, a freezing level detector circuit 766 parallel to thecomparator circuit 762 discriminates whether the ambient temperature T₀of the thermosensitive resistor 750 is below 0° C., which causes frostto develop.

To the output circuit 770 serving as the output means for theconstant-current variable circuit 776, the amplifier 778 and thearithmetic circuit 760, the timing signal generator circuit 782 suppliesa signal to alternately change over the current values between i_(A),i_(B) to be supplied to the thermosensitive resistor 750, another signalto do that change-over in response to the last-named signal, and stillanother signal to do the output control, each signal at a predeterminedperiod.

If the temperature increase is below a reference value, a discriminator768 judges it as a sign of the developing of frost or dew, based on theoutput of the comparator circuit 762. To detect the developing of frostonly, it requires an additional condition that an ambient temperature T₀is below 0° C.

FIG. 8 shows the perspective appearance around the thermosensitiveresistor 750 in the practical sensor of FIG. 7. In FIG. 8, thethermosensitive resistor 750 is carried on a distal end of a shaft 752extending through a side wall of a base 746 in the form of an angled orhook-shaped plate.

With no frost or dew around, as shown in FIG. 9, the thermosensitiveresistor 750 is located in air. With frost or dew around, as shown inFIG. 10, the surface of the thermosensitive resistor 750 is covered withfrost or dew.

The operation of the sensor of FIG. 7 (third embodiment) will nowdescribed in greater detail with reference to FIG. 11 showing apractical circuit.

In FIG. 11, to the thermosensitive resistor 750, a comparative referencecurrent i_(A) and a heat-generating current i_(B) are alternatelysupplied at chronologically different timings from the constant-currentvariable circuit 776 which is composed of the transistors Q₁, Q₂.

To make a change-over on the constant-current variable circuit 776, thesensor is equipped with a timing circuit 782. In the illustratedembodiment, the timing circuit 782 includes an oscillator 784 and a gate786; the respective supply timings of the two current values i_(A),i_(B) are alternately changed over in an oscillation period of theoscillator 784.

The oscillator 784 supplies an oscillation output from the gate 786 tothe collector of each of the transistor Q₁, Q₂ via a resistance 788.When the output of the oscillator 784 is "L" level, the current flowingto the collector of the transistor Q₂ via the resistance 788 increases.At that moment, since the transistors Q₁, Q₂ jointly constitute aso-called current mirror, the initial current value i_(A) increases toreach the heat-generating current i_(B) as the current i_(A) flowsthrough the collector of the transistor Q₂.

The output side of the thermosensitive resistor 750 is connected to thenegative input side of the comparator circuit 762, as described above;when the comparative reference current i_(A) is supplied to thethermosensitive resistor 750, then its output is supplied to theamplifier 778.

The amplifier 778 is composed of an amplifier circuit 790 and a switch792 disposed immediately upstream of the amplifier circuit 790. When theoutput of the oscillator 784 is "L" level, the timing circuit 782 issuesa change-over signal to open the switch 792.

Accordingly, the amplifier 778 can fetch the output of thethermosensitive resistor 750 only when the comparative reference currenti_(A) is supplied to the thermosensitive resistor 750.

The output of the amplifier 778 is supplied to the holding circuit 780,and the output of the holding circuit is supplied to the positive inputterminal of the comparator 762.

Thus the comparator 762 makes a delay, by the holding circuit 780, ofone of the outputs of the thermosensitive resistor 750, when thecomparative reference current i_(A) and the heat-generating currenti_(B) are supplied to the thermosensitive resistor 750 at thechronologically different timings, to meet the respective timings forcomparison.

In the amplifier 778, its amplification constant is set such that thesignal values to be supplied to the comparator 762 are identical when nofrost or dew exists around the thermosensitive resistor 750.Consequently the output of the comparator 762 is 0 (zero) during nodeveloping of frost or dew.

On the contrary, when frost or dew develops over the surface of thethermosensitive resistor 750, the negative input value of the comparatorcircuit 762 descends. A signal corresponding to the difference intemperature increase due to the heat-generation is supplied from a firstcomparator 794 to a second comparator 796 where the output value of thefirst comparator 794 is compared with a reference value. Then from thesecond comparator 796, a frost-and-dew signal is outputted when thedifference in temperature increase due to heat radiation is over apredetermined value.

Thus in this embodiment, the sensor is also equipped with a freezinglevel detector circuit 766 to obtain a signal when frost has developsover the thermosensitive resistor 750. In the freezing level detectorcircuit 766, the output of the holding circuit 780, namely, the outputof the thermosensitive resistor 750 during supplying the comparativereference current i_(A) is compared with the reference value. When theambient temperature (thermistor) descends below 0° C., then the frostsignal is outputted.

Further, these two detection signals take their AND output by thediscriminator circuit 768 including the gate 798, and are then suppliedto a non-illustrated processing circuit via the output circuit 770.

Here, during the comparative reference current i_(A) is being suppliedto the thermosensitive resistor 750, the negative input value of thefirst comparator 794 is lower than the positive input value by 1/α. Thisdifference is outputted to the first comparator 796, which might causean error frost-and-dew signal.

A switch 800 is connected to the output side of the discriminator 768.To this switch 800, a timing signal is supplied from the oscillator 784in the timing circuit 782 via a gate 802 built in the timing circuit782. By releasing the switch 800 when the comparative reference currenti_(A) flows to the thermosensitive resistor 750, it is possible to avoidsuch a misoperation.

Connected to the downstream side of the switch 800 are two transistorsQ₃, Q₄, which constitute, jointly with the switch 8000, the outputcircuit 770. Vcc is connected to the collector and base of each of thetwo transistors Q₃, Q₄. These transistors Q₃, Q₄ serve to amplify theoutput of the discriminator 768 and deliver this amplified output as afrost-and-dew notifying signal to a subsequent processing circuit.

In this illustrated embodiment, the principles of this invention isapplied for the purpose of detecting frost only. Alternatively, byomitting or modifying each of the foregoing threshold levels, it ispossible to detect both frost and dew, or only dew.

According to this embodiment, partly since the thermosensitive resistorused as a frost and dew detecting means is hardly subjected to theexterior force or vibration, and partly since the thermosensitiveresistor is inexpensive and small-sized and has an excellent mechanicalstrength, it is possible to detect the developing of frost or dew withvery high reliability.

FIG. 12 shows a fourth embodiment, in which an output port signal of aμ-CPU 906, instead of the oscillation circuit, is used to instruct theconstant-current variable circuit 876 to change over the current to besupplied to the thermosensitive resistor 850. The detected temperaturesignal from a thermosensitive resistor 850 is digitalized by an A/Dconverter 904 and is then processed in the μ-CPU 906. The result ofarithmetical operation in the μ-CPU 906 is outputted as a frost-and-dewsignal from an output circuit 870.

Apart from the arithmetical operation of the detection signals, thisembodiment is identical in the basic principle of frost and dewdetection with the third embodiment, giving the similar result.

To sum up the foregoing description, according to the first and secondembodiments, partly because a pair of thermosensitive resistors is usedas a frost and dew detecting means, and partly because the change of adifferential temperature between the thermosensitive resistors is usedas a parameter for the arithmetical operation, occurrence ofmisdetection due to any outside force or vibration can be reduced to aminimum, thus guaranteeing an inexpensive frost and dew sensor whichoffers a very reliable performance and has an adequate mechanicalstrength.

According to the third and fourth embodiments, partly since aheat-generating current and a comparative reference current arealternately supplied to a single thermosensitive resistor such as athermistor to create a differential temperature, and partly since thedeveloping of frost or dew is detected in terms of the reduceddifferential temperature due to the action of heat conducting when frostor dew has developed over the surface of the thermosensitive resistor,it is possible to achieve an improved frost and dew sensor which issmall-sized, durable and inexpensive and offers an reliable detectionperformance. Further, sensible detection of frost and dew can beachieved without causing any misoperation due to the outside force orvibration.

What is claimed is:
 1. A frost and dew sensor comprising:(a) athermosensitive resistor capable of generating heat by itself due to agiven current supplied thereto, said thermosensitive resistor having aresistance varying according to the change of temperature of saidthermosensitive resistor; (b) a power source for changing thetemperature of said thermosensitive resistor periodically, said powersource including a variable constant-current source for alternatelysupplying a heat-generating current and a comparative reference currentto said thermosensitive resistor, said comparative reference currentbeing such that it causes a temperature increase of said thermosensitiveresistor to only a negligible extent with respect to the temperatureincrease due to the heat-generating current; and (c) an arithmeticcircuit for fetching a temperature of said thermosensitive resistor asan output voltage according to a resistance corresponding to thetemperature and for generating a frost-and-dew signal according to adifference between the output voltage during the heat-generating currentis being supplied to said thermosensitive resistor and the outputvoltage during the comparative reference current is being supplied tosaid thermosensitive resistor, whereby the frost-and-dew signal isissued as presumed that said thermosensitive resistor is covered withfrost or dew when a differential temperature of said thermosensitiveresistor in such two durations is lowered.
 2. A frost and dew sensoraccording to claim 1, wherein said arithmetic circuit includes:anamplifier for amplifying the output voltage of said thermosensitiveresistor, during the comparative reference current is being suppliedthereto, at a rate corresponding to a ratio of the heat-generatingvoltage to the comparative reference current to output the amplifiedoutput voltage; a holding circuit for holding the output of saidamplifier to output the same output with a delay for the duration thecomparative reference current is being supplied to said thermosensitiveresistor; and a comparator circuit for first comparing the output ofsaid holding circuit with the output voltage of said thermosensitiveresistor, during the heat-generating current is being supplied, to finda differential temperature of said thermosensitive resistor between thetemperature during the comparative reference current is being suppliedand that during the heat-generating current is being supplied, and forsecondly comparing the differential temperature with a predeterminedreference value.
 3. A frost and dew sensor according to claim 1, whereinsaid arithmetic circuit includes:an A/D converter for converting theoutput voltage of each said thermosensitive resistor into digital data;a μ-CPU for performing a predetermined arithmetical operation withrespect to the digital data, outputted from said A/D converter, to finda differential temperature at said thermosensitive resistors; and anoutput circuit for generating and outputting a frost-and-dew signalaccording to the arithmetic result of said μ-CPU.
 4. A frost and dewsensor according to claim 2, wherein said comparator circuit includes:afirst comparator for comparing the output signal of said thermosensitiveresistor, during the heat-generating current is being supplied, with theoutput of said holding circuit to find the difference between the twooutputs; and a second comparator for comparing the output signal of saidfirst comparator with a reference value.
 5. A frost and dew sensoraccording to claim 4, wherein said arithmetic circuit includes afreezing level detector circuit for comparing the output current of saidthermosensitive resistor corresponding to the comparative referencecurrent with a predetermined freezing level reference value to detectwhether an ambient temperature of said thermosensitive resistor is belowthe freezing point.
 6. A frost and dew sensor according to claim 5,wherein said arithmetic circuit includes a discriminator for comparingthe output signal of said comparator circuit with the output signal ofsaid freezing level detector circuit to discriminate whether saidthermosensitive resistor is covered with frost, when the ambienttemperature of said thermosensitive resistor is below the freezingpoint.
 7. A frost and dew sensor according to claim 6, wherein saidarithmetic circuit includes an amplifier for amplifying the output ofsaid discriminator to output the amplified output;said power sourceincluding an oscillator for producing an alternate timing to alternatethe output of said variable constant-current source between thecomparative reference current and the heat-generating current, saidoscillator including a timing circuit for supplying a control signal tosaid amplifier according to the alternate timing produced by saidoscillator and also for supplying the same control signal to said outputcircuit according to the same alternate timing; said amplifier circuitincluding a switch adapted to be energized and deenergized by saidtiming circuit in such a manner that the output current of saidthermosensitive resistor is inputted only during the comparativereference current is being supplied to said thermosensitive resistor;and said output circuit including another switch adapted to be energizedand deenergized by said timing circuit in such a manner that the outputof the frost-and-dew signal to the exterior is prohibited during thecomparative reference current is being supplied to said thermosensitiveresistor.
 8. A frost and dew sensor according to claim 7, furtherincluding a shaft carrying thereon said thermosensitive resistor, and abase supporting said shaft, said thermosensitive resistor being athermistor.